Toyohiko Yano

Synthesis and high temperature mechanical properties of Ti3SiC2/SiC composite

A high density Ti3SiC2/20 vol % SiC composite was hot pressed under a uniaxial pressure of 45 MPa for 30 min in an Ar atmosphere at 1600 °C. The grain size of the Ti3SiC2/SiC composite was finer than that of monolithic Ti3SiC2, though the composite was hot pressed at a higher temperature, due to the dispersion of SiC particles in the Ti3SiC2 matrix. Room temperature fracture toughness of the composite and Vickers hardness were measured as 5.4 MPa m1/2 and 1080 kg mm−2, respectively. A higher flexure strength of the composite compared to that of monolithic Ti3SiC2 was measured both at room temperature and up to 1200 °C. At 1000 °C, the composite showed a lower oxidation rate than that of monolithic Ti3SiC2.

High-pressure sintering of cBN-TiN-Al composite for cutting tool application

Abstract Microhardness, phase combination and microstructure of the c(cubic)BN–TiN–Al composite sintered under static pressure of 5.8 GPa and at temperatures of 1200 and 1400 °C for 30 min were investigated. Microhardness increased with increasing cBN content of the specimens sintered at both temperatures, and the highest microhardness of 30.7 GPa was achieved for the 75 vol.% cBN–13 vol.% TiN–12 vol.% Al composition specimen sintered at both 1200 and 1400 °C. Most of the samples sintered at 1200 °C had higher microhardness than the same composition specimen sintered at 1400 °C. During high-pressure sintering, reactions occurred between cBN, TiN and Al, then new compounds, TiB2 and AlN, were formed. TEM analysis indicated that all cBN and TiN grains were surrounded with an AlN layer of different thickness, and small TiB2 and AlN grains were co-existed to form clusters.

High-resolution electron microscopy of neutron-irradiation-induced dislocations in SiC

Abstract Neutron-irradiation-induced dislocations in β- and α-silicon carbide (SiC) were observed using a high-resolution electron microscope. In β-SiC, small planar defects about 20 nm in diameter lying on {111} planes were determined as interstitial Frank loops, having a Burgers vector b=⅓〈111〉. On the basis of image simulation by the multislice method, it was determined that the loops consist of an insertion of a single Si-C layer into {111} stacking to create two rotated layers. These loops were induced by heavy neutron irradiation doses of above approximately 5 × 1026 neutrons m−2 (E>0.1 MeV) in a fast reactor. Defect nuclei a few nanometres in diameter in hexagonal α-SiC were induced by lower doses in a thermal reactor (2 × 1025 neutrons m−2) (E > 0.1 MeV). They are on the (0001) basal plane and have a Burgers vector b=⅙[0001].

High-resolution electron microscopy of a SiC/SiC joint brazed by a Ag-Cu-Ti alloy

A high-resolution electron microscope observation (HREM) was performed on the joined portion of a brazed polycrystalline or single crystal SiC to itself with (Ag-28wt% Cu) + 2wt% Ti alloy foil. The brazing was done under vacuum at temperatures of 800° C to 950° C with a holding period of up to 30 min. Reaction products formed at the joined interface were found to be mainly TiC. In the specimen brazed at 800° C with the holding time of 0 min, reaction product TiC formed itself into small crystallites with a diameter of less than 20 nm, and an amorphous like layer was found between SiC and TiC. On the other hand, TiC was formed as a layer along the joined interface for the specimen brazed at 950° C for the holding time of 30 min. Lattice matching of SiC to TiC crystals appeared to be good so the high bonding strength of the joint was attributed to the formation of this epitaxial interface between SiC and TiC.

Preparation of Silica Microspheres Containing Ag Nanoparticles

Two sol-gel fabrication processes were investigated to make silica spheres containing Ag nanoparticles: (1) a modified Stöber method for silica spheres below 1 μm size, and (2) a SiO2-film formation method on spheres of 3–;7 μm size. The spheres were designed to incorporate silver nanoparticles of high χ(3) in a spherical optical cavity structure for the resonance effect. For the incorporation, interaction between [Ag(NH3)2]+ ion and Si-OH was important. In the Stöber method, the size of the silica spheres was determined by a charge balance of plus and minus ions on the silica surface. In the film formation method, the capture of Ag complex ion on the silica surface depended on whether the surface was covered with OH groups or not. After doping [Ag(NH3)2]+ into silica particles or SiO2 films on the spheres, these ions w ere reduced by NaBH4 to form silver nanoparticles. From plasma absorption at around 420 nm wavelength and TEM photographs of nanometer-sized silver particles, their formation inside the spherical cavity structures was confirmed.

Microstructure and mechanical properties of RB-SiC/MoSi2 composite

Microstructure, high temperature strength and oxidation behaviour of reaction bonded silicon carbide, RB-SiC/17 wt% MoSi2 composite prepared by infiltrating a porous RB-SiC bulk (after removal of free silicon) with molten MoSi2 were investigated. There was good bonding between the SiC and MoSi2 particle, without a significant reaction zone and microcracking caused by the thermal mismatch stresses. A thin (∼2 nm) layer, however, was observed at the SiC/MoSi2 interfaces. At room temperature, the composite exhibited a bending strength of 410 MPa, which is ∼20% loss in comparison to that of RB-SiC alone (containing ∼ 10 wt% free silicon). However, the composite strength increased to a maximum of 590 MPa in the temperature range 1100 and 1200° C and dropped to 460 MPa between 1200 to 1400° C, after which the strength remained constant. The passive oxidation of the composite in dry air in the temperature range 1300 to 1400° C was found to follow the parabolic rate law with the formation of a protective layer of cristobalite on the surface.

X-ray diffractometry and high-resolution electron microscopy of neutron-irradiated SiC to a fluence of 1.9×1027 n/m2

Abstract Neutron-induced damage in SiC up to a fluence of 1.9×10 27 n/m 2 ( E >0.1 MeV) was examined by means of X-ray diffractometry and high-resolution electron microscopy. Specimens of β-SiC were irradiated in fast breeder reactors at 370 to 650°C. The lattice parameters of all specimens were increased by the irradiation, but above 2×10 26 n/m 2 lattice expansion decreased and was accompanied by significant peak broadening. Electron microscopy revealed a high density of interstitial loops on {111}, where X-ray diffraction peaks showed marked broadening. Peak broadening could be attributed mainly to the crystallite size effect at lower fluences, but a strain contribution was significant above 2×10 26 n/m 2 . Electron diffraction patterns and high-resolution images indicated preservation of crystallinity up to the highest fluence observed. Thermal annealing up to 1000°C did not affect the peak broadening and average loop diameter. Above 1400°C, decrease in lattice strain and increase in crystallite size with increasing annealing temperature were observed.

Improvement of the mechanical properties of hot-pressed silicon-carbide-fiber-reinforced silicon carbide composites by polycarbosilane impregnation

Abstract Green sheets of SiC with Al 2 O 3 –Y 2 O 3 –CaO sintering additives prepared by the doctor-blade method and polycarbosilane (PCS)-impregnated Hi-Nicalon cloth with a BN coating were used for the fabrication of SiC-fiber-reinforced SiC (SiC/SiC f ) composites by hot-pressing. Two kinds of SiC/SiC f composites with different fiber volume fractions were fabricated and their room-temperature mechanical properties were investigated. These composites showed non-brittle fracture behavior. The maximum strength of a composite with 52 vol.% of fibers was about twice as high as that of a composite with 40 vol.% of fibers, and the composite hot-pressed at 1700°C showed the highest maximum strength. In this fabrication process, PCS-impregnation into Hi-Nicalon cloth was an effective way of forming the matrix between fibers.

High pressure sintering of diamond-SiC composite

Sintered polycrystalline compacts in the system diamond-10–50 wt% SiC having average grain size of less than 1 μm were prepared at pressure of 6 GPa and temperature between 1400 and 1600 °C. Knoop indentation hardness of the compacts increased with diamond content and sintering temperature, and specimens with a Knoop indentation hardness greater 40 GPa were obtained. It was found that small amount of Al addition into the starting diamond-SiC powder was effective to improve relative density and Knoop indentation hardness of the compacts. The formation of graphite was also suppressed by the addition of Al. Microstructure observation by SEM and TEM suggested that Al segregated at the grain boundary and promoted the bonding between grains. Thin microtwins were observed in diamond grains, whereas fine wavy structures with slightly different orientations were observed in SiC grains, with or without Al addition.

Physical property change of heavily neutron-irradiated Si3N4 and SiC by thermal annealing

Abstract Changes in macroscopic length, lattice parameter and thermal diffusivity of neutron-irradiated Si3N4 and SiC ceramics up to a fluence of 4.2×10 26 n / m 2 were measured. Macroscopic length increase of Si3N4 was almost one half of that of SiC. Thermal diffusivity of both ceramics was reduced severely by the irradiation at 390–540°C. Slight increase in the a-axis and slight decrease in the c-axis lattice parameter were detected for Si3N4. The amount of lattice parameter change of Si3N4 was very small compared with the macroscopic length change. Changes in these properties due to post-irradiation thermal annealing up to 1500°C were measured. Large part of thermal diffusivity of Si3N4 was recovered by annealing, with small step at ∼1100°C, but macroscopic length did not significantly change by annealing. Change in lattice parameter showed a complicated trend. It is supposed that formation of interstitial loops on the planes parallel to the c-axis, formation of voids during annealing or difficulty of recovery of points defects/loops, or solid solution formation due to glassy grain boundary phase may influence the recovery behavior of Si3N4 ceramics. Changes in macroscopic length, lattice parameter or thermal diffusivity of SiC by annealing coincided with the results of previous works. The critical irradiation conditions for loop formation/XRD line broadening for SiC is discussed based on the present and previous results.